The present invention relates to dynamic weight measurement, and particularly to a continuously moving weigh conveyor for weighing individual quantities of product to be packaged.
Many different kinds of food loaves are produced in a wide variety of shapes and sizes. Meat loaves consisting of ham, pork, beef, lamb, turkey, fish and other meats have been commercialized. Such meat loaves or cheese loaves or other food loaves are commonly sliced and collected in groups in accordance with a particular weight requirement, the groups being packaged and sold at retail. The number of slices in a group may vary depending on the size and consistency of the food loaf. For some products, neatly aligned stacked sliced groups are preferred, while for other products the groups are shingled so that a purchaser can see a part of every slice through transparent packaging. For bacon or other food products of variable shape, the slicing and packaging problems are more challenging.
To properly allocate a sufficient number of slices or a sufficient overall weight of the group of slices, a weighing operation is undertaken in line with the slicing operation. This is particularly advantageous in the application of high-speed slicers employed in meat processing plants.
Some known high-speed food loaf slicing machines are disclosed, for example, in U.S. Pat. Nos. 5,566,600; 5,704,265; and 5,724,874.
It is known to weigh a stack of sliced product transported on a conveyor from a slicing operation. Such a xe2x80x9ccheck-weighingxe2x80x9d operation is disclosed, for example, in U.S. Pat. Nos. 3,846,958 and 4,065,911. However, in order to make such a measurement on a dynamic-weigh basis, the prior art weigh scale methods utilize an optical or other external triggering device to activate the weigh scale for choosing an accurate sample period. The sample period is set on a fixed timing basis from the trigger of the triggering device.
The present inventor has recognized the desirability of providing a dynamic-weigh checker for a conveyed series of products, or groups or stacks of products, which does not rely on an external triggering device to ascertain the correct sample period of the product, or groups or stacks of products, moving over the associated weigh scale.
The present invention provides a data acquisition and/or control device for a conveyor weigh scale or xe2x80x9cweigh scale controlxe2x80x9d and a method for operating a conveyor weigh scale that automatically determines the correct sample period for accurately weighing product carried over the weigh scale. The present invention provides an algorithm for effective data acquisition and/or control associated with such a weighing operation. The weigh scale and control of the present invention can advantageously be configured to be combined with a high speed slicing apparatus and can give feedback on product output weight to be used as a control parameter for the slicing apparatus.
According to the invention, a conveyor weigh scale senses a dynamic weight of product as it passes over the weigh scale. This dynamic weight can be expressed as a weight waveform of sensed weight over time as the product passes over the weigh scale. An accurate weight reading for a moving product can be made only during a brief sample period within the waveform, where the weight readings are substantially constant and representative of the static weight of the product. Prior known continuously moving product scales have used devices such as a laser sensor or photosensitive components to detect when a product has entered the scale and then uses fixed timing numbers to estimate the position of the sample period on the weight waveform to make a weight measurement.
The present invention provides a software algorithm for a weigh scale associated with a continuously moving conveyor which is capable of positioning the sample period on each product weight waveform wherein the weight and speed of the product passing over the scale does not affect the positioning of the sample period. The sample period is calculated mathematically using the slope characteristics of the waveform.
The algorithm first looks for a minimum preselected positive amount of weight deviation to activate or establish a xe2x80x9ctriggerxe2x80x9d. A first inflection point, that point where the rate of weight change over time dW/dt (the slope of the waveform), first decreases; i.e., the waveform changes from a more positive slope to a less positive slope, is determined. The slope at the first inflection point is recorded and defined as the xe2x80x9cmaximum positive slopexe2x80x9d dW1/dt. Once the maximum positive slope is found, the algorithm begins recording weight samples at a sampling rate. The algorithm checks the weight waveform of sampled weights for a slope dW2/dt which is a first pre-selected percentage of the maximum slope but negative in slope value. The first pre-selected percentage is preferably about xe2x88x9250% of the maximum slope dW1/dt. This point is determined as the xe2x80x9cweight-off-scalexe2x80x9d point.
When the weight-off-scale point is reached, then the algorithm will look backward (reverse chronological order) through the saved data of weight samples to find another point which has a slope dW3/dt which is a second preselected negative percentage of the maximum positive slope dW1/dt. The second pre-selected negative percentage is preferably about xe2x88x9210% of the maximum positive slope. This point is defined as the xe2x80x9cend sample position.xe2x80x9d The end sample position is experimentally known to be on or close to a flat part of the waveform representing substantially constant weight values.
With the end sample position known, a xe2x80x9cstart sample positionxe2x80x9d is determined to fall within, or at the start of, the flat part of the waveform, such that the weight values within the sample period between the start and end sample positions are substantially constant. The start sample position can be calculated as a first point having a predetermined slope on the waveform, reviewing the weight samples in reverse chronological order from the end sample position; or can be experimentally determined to be within a preselected number of sample points in front of the end sample position. The weight values within the sample period are then averaged to determine a static weight value.
In an apparatus configured according to the invention no extra hardware cost is required for a separate triggering device, separate from the weigh scale device. The apparatus of the invention requires no adjustment for weight changes of the product moving over the scale. According to the invention, no synchronization is required between a separate triggering device and the scale device. The apparatus of the invention achieves an increased operational reliability by eliminating the need for a separate triggering device.
Another advantage of the invention is the ability of the software algorithm to compensate for product which may have a different orientation from stack to stack, either intentionally or accidentally. A narrow product, which moves onto the weigh scale with different orientations will produce longer and shorter weight waveforms. The algorithm positions the sample period from the trailing edge of the waveform which eliminates many orientation-based weighing problems experienced by trigger and fixed-timing weigh systems.
Other features and advantages of the present invention will become readily apparent from the following detailed description and the accompanying drawings.